ESD testing is sometimes a requirement for products that contain electronic components. Static electricity can commonly afflict microelectronic circuits, and the electrical surge produced by such static electricity can result in unpredictable circuit behavior. Electronic components are designed to have, and are tested to determine, a certain level of immunity to ESD. The tests address a variety of conditions that the components may encounter during packaging, handling, assembly, service, and intended operation. As such, ESD immunity is sometimes measured and tested to ensure product quality. This testing can involve applying an ESD pulse to a circuit.
Prior to applying an ESD pulse to a circuit to test that circuit, an ESD simulation pulse may be characterized to determine aspects of the pulse, including the magnitude and timing measurements of the incident ESD pulse. One of the most challenging measurements for ESD characterization has been the RC Time Constant measurement. A traditional method of determining the RC Time Constant has been to manipulate oscilloscope cursors on an electronic representation of a waveform that presents a recorded ESD signal, and to continually manipulate the electronic representation of the waveform shape and to continually reposition the oscilloscope cursors until the measurement criteria is met. This approach, however, is time consuming and can be inaccurate because of the disparity between the sampled data that comprises the ESD pulse and the pixelated representation of that data that is selected through user manipulation of the oscilloscope cursors.
International standard ISO10605:2001(E) standard (test methods for electrical disturbances from electrostatic discharge for road vehicles) describes a number of test specification requirements. Many of these test criteria are qualified through simple numerical parameter measurements. For example, a parametric measurement of the ESD pulse rise time can be obtained automatically using existing oscilloscope features by setting adjustable threshold levels on the rise time parameter to account for the characteristic exponential decay shape of an ESD pulse. Some specification requirements, however, cannot be measured using traditional oscilloscope parameters. An example such measurement is the RC time constant for an air discharge ESD simulator. The ISO10605:2001 RC time constant measurement describes a single event discharge waveform which must meet the acceptance criteria time of 600 ns±130 ns from the discharge between two sample points, having an amplitude difference of 63.2120559% along the discharge curve when using a 330 pF probe and 300 ns±60 ns from the discharge between two sample points which have an amplitude difference of 63.2120559% along the discharge curve for a 150 pF probe [1]. Note that an amplitude difference of 63.2120559% corresponds to an amplitude ratio of 0.367879441.
Determining the RC time constant measurement under such criteria is especially challenging due to measurement inaccuracy contributions from human error and error originating from the measurement system. Presently, oscilloscopes are not equipped to address this RC time constant measurement. Therefore, measurements rely on operators manually placing the cursors, which introduces multiple forms of inaccuracy. A contributor to cursor inaccuracy is the resolution of cursor position inherent to all oscilloscopes, in which the precise time position of a placed cursor is often only specified to be within 2% of the time coordinate reported by its position. The cursor placement problem is exacerbated by manual placement of the cursor, which introduces arbitrary error into the selected time coordinate, and which results in measurement bias. In addition to the compounding error described above, a lack of statistical significance occurs when these measurements are obtained by a human instead of being performed according to an automated mechanism, which results in a loss of both precision and accuracy. Moreover, engineering personnel, equipment, and test facility time is consumed when individuals attempt to perform such measurements manually.